1. Field of the Invention
The present invention relates to a wire bonding apparatus that includes a function for countering the reaction force applied to a bonding head when it is moved horizontally.
2. Description of the Related Arts
FIG. 1 is a plan cross-sectional view of a conventional wire bonding apparatus, and FIG. 2 is a side cross-sectional view of the wire bonding apparatus in the direction C in FIG. 1. In FIGS. 1 and 2, an XY stage 1 is moved horizontally (the X and Y axial directions in FIG. 1) by motors 2 and 3. That is, the XY stage 1 is moved in the Y axial direction by the motor 2, while it is moved in the X axial direction by the motor 3. A bonding head unit 4 is fixed to the XY stage 1.
A bonding arm 6 is provided on the bonding head unit 4 to move along the Z axis at its end perpendicular to the face of the XY stage 1, i.e., perpendicular to the direction in which the XY stage 1 is moved, so that the bonding arm 6 can pivot at a rotary shaft 5. The rotary shaft 5 passes through and is fixed to the center of the bonding arm 6, while both sides of the rotary shaft 5 are supported by the side walls of the bonding head unit 4.
A capillary 7 is attached, via a horn 6-1, to the distal end of the bonding arm 6. A magnet 8-1 for a linear motor 8 is provided at the rear of the bonding arm 6, while a magnetic circuit 8-2 for the linear motor 8 is attached to the bonding head unit 4 and encloses the magnet 8-1.
An encoder 10 is located at one end of the rotary shaft 5 that passes through and is fixed to the bonding arm 6. The encoder 10 detects at the rotary shaft 5 the pivoted position of the bonding arm 6, i.e., the position in the Z axial direction of the bonding head 9 that is constituted by the horn 6-1 and the capillary 7.
A cut clamp attachment arm 11 is attached to the bonding arm 6, and extends out above the bonding head 9. A cut clamp 12 is attached at the distal end of the cut clamp attachment arm 11, and a bonding wire (not shown) leading to the capillary 7 is inserted into the cut clamp 12.
FIG. 3 is a block diagram showing the essential portion of an electronic circuit in a control unit attached to the wire bonding apparatus. A control unit 13 comprises a controller 13-1, an operating unit 13-2 and an amplifier 13-3. The current position of the bonding head 9 in the Z axial direction is transmitted from the encoder 10 to the controller 13-1. The operating unit 13-2 calculates the next position in the Z axial direction to which the bonding head 9 is to be moved, and transmits the position to the controller 13-1.
The controller 13-1 generates a control signal (torque instruction value) by using the current position of the bonding head 9 in the Z axial direction, which is received from the encoder 10, and the next movement position, which is received from the operating unit 13-2, and transmits the control signal to the amplifier 13-3. The amplifier 13-3 amplifies the control signal received from the controller 13-1, and transmits the resultant control signal as a drive signal to the linear motor 8. The linear motor 8 is actuated upon the reception of the drive signal from the controller 13-1 via the amplifier 13-3, and provides a thrusting force at the rear of the bonding arm 6. In response to this thrusting force, the bonding arm 6 pivots at the rotary shaft 5, and the bonding head 9 is moved vertically.
FIG. 4 is a diagram showing the processing during which the wire bonding apparatus actually performs wire bonding. In FIG. 4, the bonding head 9 rapidly drops at time t1, and when the bonding head 9 has reached the search level, the dropping speed is reduced to a low level (t2 to t3), and US bonding is performed (t3 to t4). Then, the bonding head 9 is raised and the reverse operation is performed (t4 to t5), and a loop-up operation is started (t5 to t6). When the bonding head 9 is continuously raised and reaches a predetermined height (t6 to t7), the loop-down operation is performed (t7 to t8), the bonding head 9 is moved to the lead frame side as the XY stage 1 is moved horizontally, the search operation is again initiated (t8 to t9), and the US bonding is performed (t9 to t10). Following this, the bonding head 9 is raised (t10 to t11), and cut clamp 12 is closed while the bonding wire 15 such as a thin gold wire is cut off (t11 to t12). Then, the distal end of the wire is heated by a voltage discharge and forms a ball (t12 to t13). Thereafter, the cut clamp 12 is opened, and the operation at t1 to t13 is repeated.
However, in the conventional wire bonding apparatus, accurate loop control of the wire along the locus that is prepared in advance can not be exercised, although vertically positions (along the Z axis) of the bonding head 9 before it is moved to the lead frame side (in the XY axial direction) is setting.
This problem occurs because the center of gravity W1 of the bonding arm 6 does not correspond to the center S1 of the rotary shaft 5, which is the pivoting center of the bonding arm 6, and because the moment M1 occurs due to the shifting of the center of gravity when the bonding arm 6 is rapidly moved in the XY axial direction. That is, when the center of gravity W1 of the bonding arm 6 is shifted in position relative to the center Si of the rotary shaft 5, the moment M1 occurs around the rotary shaft 5 due to the acceleration of the axial directional element Y when the XY stage 1 is moved. Because of the occurrence of this moment M1, an unpredicted large reaction force F1 is applied to the bonding head 9, and this reaction force F1 interferes with the positioning of the bonding head 9 along the Z axis.
To resolve this shortcoming, it is one object of the present invention to provide a wire bonding apparatus that reduces interference with the positioning control of the bonding head in the perpendicular direction (along the Z axis) when it is moved horizontally (along the X and Y axes), and that provides accurate loop control for a wire.
To achieve the above object, according to the present invention, a reaction force applied to a bonding head when it is moved horizontally is calculated; the obtained reaction force is used to correct a torque instruction value that is transmitted to means for driving a bonding arm; and a thrusting force is generated to counter the reaction force applied to the bonding head.
With this arrangement, when the bonding arm is moved horizontally (along the X and Y axes), the reaction force, which is applied to the bonding head due to the acceleration generated at this time, is countered, and interference with the positioning control of the bonding head in the perpendicular direction (along the Z axis) is reduced.
According to the present invention, the reaction force applied to the bonding head is calculated based on the distance between the center of the rotary shaft and the center of gravity of the bonding arm, the mass of the bonding arm, and the acceleration generated when the stage is moved horizontally.
Furthermore, a motor (e.g., a linear motor) is used as means for driving the bonding arm. The reaction force applied to the bonding head is calculated based on the value of a current that flows across the motor, or based on the torque instruction value, which is transmitted to the means for driving the bonding arm, and the current position of the bonding head (the position along the Z axis). In this invention, the reaction force calculation method is not limited to the above method, and the motor that can be used is not limited to a linear motor.